Chemical Oxygen Demand

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Contents

- What is Chemical Oxygen Demand? - Why is Chemical Oxygen Demand important? - Measuring Chemical Oxygen Demand - Typical applications

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What is chemical oxygen demand?Chemcial Oxygen Demand (COD) measures the organic matter in a sample that is oxidized by a strong oxidizing agent.

It is used to measure the number of pollutants in water, as it measures the quantity of chemicals in the water that are not oxidized by biological organisms in the water.

Organic matter consists of carbon compounds and during the oxidation process, these compounds are oxidized to give CO2 and H2O.

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Why is chemical oxygen demand important?Aquatic organisms need oxygen to survive, so it is essential that there is enough dissolved oxygen in water to ensure these organisms can survive.

Pollutants in the water, normally organic matter, that can be oxidized will consume dissolved oxygen in the water.

In order to ensure the dissolved oxygen levels in the water are maintained, it is essential to monitor the COD to determine the levels of pollutants in the water, and hence be able to remove these pollutants to minimize the COD.

In many countries, there is a restriction on the maximum COD levels allowed for wastewater to be released into the environment.

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Measuring chemical oxygen demandCOD is measured using a colorimetric method. Potassium dichromate is added to the sample, and the oxidizable organic compounds in the sample reduce the dichromate ions to chromic ions. This results in a colour change from orange to green.

The quantity of remaining dichromate ions is measured, and this allows for the determination of how much potassium

dichromate has been consumed, and hence the chemical oxygen demand level is measured.

In order to accurately measure the COD levels, the sample must first have a digestion period, where it is heated with the dichromate reagent for approximately 2 hours, providing the time and energy for all the oxidizable organic matter to be oxidized.

HI-83099COD and Multiparameter Bench Photometer

HI-8309925 Vial Thermo-Reactor

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Typical applicationsWastewater - At wastewater treatment plants, the COD level in the water is monitored. Both the influent and effluent wastewater is tested to determine the COD levels before and after treatment, and hence ascertain the quality of the treatment process.

- Wastewater must be treated before being released into the environment to ensure the COD levels are below the maximum allowed COD level.

Agriculture and animal waste runoff - Agricultural wastewater can contain organic chemicals, for example from fertilizer or animal waste. This can run into bodies of water in the environment, resulting in high COD levels, which can be damaging to aquatic life.

- The COD levels must be monitored, and treated in order to minimize them and so allow for maximum growth and life of aquatic organisms.